Real-time communications devices and data processing devices are increasingly being networked by data packet networks, such as local area networks (LAN), ATM networks (ATM: Asynchronous Transfer Mode), the Internet or other data networks based on the Internet Protocol (IP). Many communications or data processing applications require the current time or some other accurate time reference. Real time clocks (RTC) or crystal-controlled timers are provided for this purpose in most communications or data processing devices.
In a networked system, the real time clocks of the networked communications and data processing devices must be synchronized in order to provide a standard time reference throughout the network. The respectively required synchronization accuracy in this case depends essentially on the nature of the applications which are distributed via the network. While pure data processing applications generally result in synchronization requirements which are not very stringent, high synchronization accuracy is generally required for real-time communications applications. A high level of clock synchronicity is required in particular for connection-oriented communications technologies which are actually synchronous and which at present are being increasingly ported onto asynchronous data packet networks.
Synchronization of a number of real-time clocks or timers in a network requires that the time information be transmitted as accurately as possible between the network devices to be synchronized.
In the past, the so-called NTP protocol (Network Time Profile) has frequently being used for synchronization of network devices in data packet networks. The NTP protocol is described, for example, in the document “Request For Comments 1305: Network Time Protocol (Version 3), Specification, Implementation and Analysis”, by David L. Mills dated March 1992. Within the NTP protocol, time information originating from a time information source is transmitted within data packets via the data packet network. The synchronization error resulting from transmission delays can be reduced by repeatedly transmitting time information at predetermined time intervals and, possibly, in different directions. However, the synchronization accuracy of the NTP protocol is restricted by the delay time fluctuations, which are unavoidable in networks based on the Internet Protocol, and fluctuations in the packet processing times in network devices, such as routers or switching nodes. Fluctuations such as these are typically in the range from 1 to 100 milliseconds. Thus, in a good situation, a synchronization accuracy of only about 1 millisecond can be achieved by the NTP protocol. However, this accuracy is inadequate for many real-time network applications.